The seventh day Sabbath, which was from sundown Friday to sundown Saturday, was part of the Covenant (the Old Covenant) God made with the Israelites at Mount Sinai.  Under the New Covenant which God has made with the Christian Church, the seventh day Sabbath is the only one of the Ten Commandments not reinstated.¹  

From the earliest days of the Christian Church, it has been the practice of believers to meet together on the first day of the week (Sunday, the Lord's Day.)²  This is done in remembrance of the Lord Christ Jesus' resurrection from the grave on the first day of the week.

In the New Testament of the Bible, we see several examples of this: the church in Corinth and the churches of Galatia met together on the first day of every week.  The church in  Troas, also, came together on the first day of the week.³

God has placed no restrictions on which day a church is to come together in worship (all days are holy unto the Lord.)⁴  It's up to the local church as to which day they, as a body of baptized believers, choose to meet together in corporate worship.

Bible References:

¹ (Colossians 2:16; Acts 15)

² (Revelation 1:10)

³(1 Corinthians 16:1-2; Acts 20:6-7)

⁴ (Romans 14:5)